Instant output
Press Run (or Ctrl+Enter) and see the result in the output panel immediately — no page reloads.
Free JavaScript sandbox
JavaScript compiler online — run and test JS instantly
FastCompiler's JavaScript compiler lets you write and test JS snippets with live DOM output — the fastest way to debug a function, an event handler, or an async flow without touching a file manager.
Features
DOM included
Test code against real HTML and CSS tabs, not just a blank console — perfect for UI logic.
Modern JavaScript
Arrow functions, destructuring, async/await, template literals — everything your browser supports.
Private sandbox
Your code runs only on your device. No upload, no tracking, no account.
Auto-save
Snippets are saved to local storage automatically, so they survive accidental refreshes.
Export anytime
Copy the combined source or download it as a single HTML file to share or deploy.
Great things to test here
Event handlers
Wire up clicks, key presses and form submits and watch the DOM update live as you interact.
Array & object methods
Quickly verify map, filter, reduce and destructuring logic before pasting it into your project.
Async functions
Experiment with promises, async/await and setTimeout behavior in a safe sandbox.
Copy-paste examples
3 JavaScript examples to try right now
Paste each snippet into the script.js tab above and press Run. All three run 100% in your browser — no account, no upload. By Juwel, last updated September 8, 2026.
1. Counter button (events + DOM)
let count = 0;
const btn = document.querySelector('#demo-btn') || document.body.appendChild(Object.assign(document.createElement('button'), { id: 'demo-btn', textContent: 'Clicked 0 times' }));
btn.addEventListener('click', () => {
count++;
btn.textContent = `Clicked ${count} time${count === 1 ? '' : 's'}`;
});Why it matters: every interview to-do app is this pattern — state + listener + textContent update. Never use innerHTML with user input.
2. Fetch with async/await + error handling
async function loadUser(id = 1) {
try {
const res = await fetch(`https://jsonplaceholder.typicode.com/users/${id}`);
if (!res.ok) throw new Error('HTTP ' + res.status);
const user = await res.json();
console.log(user.name, user.email);
return user;
} catch (err) {
console.error('Fetch failed:', err.message);
}
}
loadUser();Tip: open DevTools console alongside the preview to see console.log. In FastCompiler the preview is a real page, so fetch behaves exactly like production.
3. Array methods interview drill
const orders = [
{ id: 1, total: 42, paid: true },
{ id: 2, total: 17, paid: false },
{ id: 3, total: 99, paid: true },
];
const revenue = orders.filter(o => o.paid).map(o => o.total).reduce((a, b) => a + b, 0);
console.log('Paid revenue:', revenue); // 141Limitations & how it compares
FastCompiler runs JavaScript in your browser's engine (V8/SpiderMonkey), not Node.js. That means window, document and fetch work, but require(), fs and npm packages do not — use the Node.js compiler for console.log-only server logic, and the HTML runner for full pages.
vs CodePen/JSFiddle: those save to the cloud and need accounts for private pens. FastCompiler auto-saves to localStorage, works offline after load, and never uploads. vs browser console: the console forgets on refresh and has no multi-file tabs — here HTML+CSS+JS persist together.
Reviewed by Juwel, founder & editor — every snippet above was run in this tool on Chrome, Firefox and Safari mobile before publishing.
Frequently asked questions
How can I run JavaScript code online for free?
Use the FastCompiler JavaScript compiler above. Write your script in the script.js tab and press Run — the result appears in the live preview instantly.
Can I test JavaScript with HTML and CSS together?
Yes. The editor includes HTML and CSS tabs, so you can test JavaScript against real markup and styles in the same page.
Does the JS compiler support modern JavaScript?
Yes. It runs in your browser's JavaScript engine, so features like arrow functions, template literals, async/await and ES modules work in any modern browser.
Is my JavaScript code uploaded anywhere?
No. Your code is compiled and executed entirely on your device. Nothing is sent to a server, and your work auto-saves to local storage.